172 OUTLINES OF PLANT LIFE. 



light reaches it chiefly from one direction. Mark certain leaves and 

 record the position of the plane of tlie blade ; 24 hours later observe the 

 position and compare vifith first. 



To show effect of direction of light as a stimulus upon stems and roots. 



Grow seedlings of white mustard thus: Tie loosely over the mouth of a 

 jelly-glass a double piece of fine bobbinet ; fill vessel with tap water to 

 the net, on which place seeds; set in dark, replacing water as it evapo- 

 rates, until seedlings are 3 cm. high, with roots as long or longer. Then 

 place in a box, blackened inside, into which light is admitted through a 

 hole 4-5 cm. in diameter, at right angles to stems and roots. Observe 

 curvatures 24 hours later. 



244. (b) Combined movements due to variations in the 

 intensity of light or heat or both are especially exhibited by 

 flowers, whose opening and closing are frequently determined 

 thereby. With some plants the predominant stimulus is 

 heat ; with others, light. Closed flowers of the tulip or 

 crocus may be made to open in 2 to 4 minutes by raising the 

 temperature 15" to 20°. The flowers of the white water-lily 

 and of the dandelion open in sunlight and close in shade. 

 By marking upon their leaves a series of equidistant parallel 

 lines with Chinese ink, and measuring later the distances to 

 which they have been spread, all such movements can be 

 clearly shown to be due to accelerated growth of the outer or 

 inner surfaces, respectively, The protection of the flower 

 parts or their proper working is secured by these movements, 

 which must not be confounded with those due to the direction 

 of light or heat rays. 



245. (c) Geotropism. — Geotropism is the state of a plant 

 or an organ when it is irritable to the force of gravity. 

 Since gravity is exerted always in the same direction, it is 

 plain that reactions to this force cannot be studied, as in the 

 case of light, by altering the absolute direction in which 

 gravity acts, but only by so changing the position of the 

 plant that the force acts in a relatively different direction. 

 The reaction to this stimulus and the fixed gravity position 

 must not be confused with the simple effect produced by the 



